Increase in midges

The wet weather in late December 2024 and January 2025 has had a positive impact on the biological health of the ponds. The volume of rain means wastewater entering the plant wasn’t as strong, and as a result, algae are flourishing.

Algae helps the pond system operate well by generating oxygen within the ponds and preventing the anoxic conditions which can cause odour. The fragile health of the ponds is demonstrated through recent odour monitoring(external link), which shows intermittent, low levels of odour are being detected at the community odour monitoring sites. This is an improvement in odour levels in late 2024, which spiked during the transition to summer mode. Maintaining the health of the ponds is critical to ensuring odour is at a minimum.

Unfortunately, algae are also a food source for midges and the increase in algae has resulted in a surge in the number of midges on and around the oxidation ponds, with numbers much higher than the previous few summers. We are aware of an increased number of reports about midge activity and are working to address the issue.

We have a midge control programme in place which includes native planting around the oxidation ponds. Native plants encourage midges to stay close to the ponds rather than spreading to local residential and commercial areas. Since 2018 we have planted approximately 75,000 native plants around the oxidation ponds.

In the past, when the ponds were healthy, we dredged the bottom of the ponds and altered the flow paths through the ponds to reduce midge numbers. We’ve considered these control options but have decided against them at this time as there is a real risk of disturbing and unsettling the health of the ponds, which could generate odour. In addition to this, odour caused by a change in pond biology takes time to address, and in a worst-case scenario, an increase in odour could last until winter.

The weather has a huge part to play in how our oxidation ponds operate, and the strength of wastewater entering the ponds is now increasing due to a recent patch of reasonably dry weather. If this weather continues it could make the pond conditions less suited to algae, which could in turn result in a decrease in midges.

We know the midges are frustrating and are grateful for the ongoing patience of residents living in impacted areas. When making decisions about the next steps we’re balancing odour and midge management to try and get the best result for the community. We’ve engaged expert external advice to get a second opinion on what the impact of control measures would be, and to ensure that we’re doing what we can.

To read the latest information on what we are doing to contol the midges and other updates on the wastewater treatment plant, head to the Christchurch Waste Water Treatment Plant page on our wesbite.

To report a problem with pests, odur or insects call 03 941 8999 (or 0800 800 169 for Banks Peninsula residents) for all incidents within Council-maintained areas.

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